# Outline of the metric system

The following outline is provided as an overview of and topical guide to the metric system:
Metric system – various loosely related systems of measurement that trace their origin to the decimal system of measurement introduced in France during the French Revolution.

## Nature of the metric system

The metric system can be described as all of the following:
* System – set of interacting or interdependent components forming an integrated whole.
** System of measurement – set of units which can be used to specify anything which can be measured. Historically, systems of measurement were initially defined and regulated to support trade and internal commerce. Units were arbitrarily defined by fiat (see statutory law) by the ruling entities and were not necessarily well inter-related or self-consistent. When later analyzed and scientifically, some quantities were designated as base units, meaning all other needed units of measure could be derived from them.

### Essence of the metric system

* International System of Units (SI) is the system of units that has been officially endorsed under the Metre Convention since 1960. Child articles are:
** SI base unit
** SI derived unit
** Non-SI units mentioned in the SI
** Metric prefixes

### Underlying philosophy

Discussions of the underlying philosophy of the metric system (and other systems of measure) include:
* Coherence (units of measurement)
* Realisation (metrology)

## Metric units of measure

Articles that exist for many units of measure that are related to various flavours of the metric system are catalogued below.
| Unit name | Quantity | SI unit | cgs unit | Other metric | Non-metric |
| abampere | electric current | | Derived metric unit with a special name | | |
| abcoulomb | electric charge | | Derived metric unit with a special name | | |
| abhenry | inductance | | Derived metric unit with a special name | | |
| abohm | electrical resistance | | Derived metric unit with a special name | | |
| abvolt | potential difference | | Derived metric unit with a special name | | |
| ampere | electric current | Base unit | | | |
| ampere-meter | magnetic pole strength | Derived coherent SI unit without a special name | | | |
| apostilb | luminance | | Derived metric unit with a special name | | |
| astronomical unit | length | Non-SI unit permitted for use with SI units | | | |
| dalton | mass | Non-SI unit permitted for use with SI units | | | |
| barye | pressure | | Derived metric unit with a special name | | |
| becquerel | radioactive activity | Derived SI unit with a special name | | | |
| bril | luminance | | | | No specified relationship |
| candela | luminous intensity | Base unit | | | |
| candela per square metre | luminance | Derived coherent SI unit without a special name | | | |
| centimetre | length | Multiple or submultiple of an SI base unit | Base unit | | |
| coulomb | electric charge | Derived SI unit with a special name | | | |
| cubic centimetre | volume | Derived coherent SI unit without a special name | Derived coherent metric unit without a special name | | |
| cubic metre | volume | Derived coherent SI unit without a special name | Derived coherent metric unit without a special name | | |
| cubic metre per second | volumetric flow rate | Derived coherent SI unit without a special name | Derived coherent metric unit without a special name | | |
| curie | radioactive activity | | Derived metric unit with a special name | | |
| day | time | Non-SI unit permitted for use with SI units | | | |
| decibel | level | Non-SI unit permitted for use with SI units | | | |
| degree Celsius | temperature | Derived SI unit with a special name | | | |
| degree of arc | angle | Non-SI unit permitted for use with SI units | | | |
| dyne | force | | Derived metric unit with a special name | | |
| electronvolt | energy | Non-SI unit permitted for use with SI units | | | |
| erg | energy | | Derived metric unit with a special name | | |
| farad | capacitance | Derived SI unit with a special name | | | |
| gal | acceleration | | Derived SI unit with a special name | | |
| gauss | magnetic flux density | | Derived metric unit with a special name | | |
| gram | mass | Multiple or submultiple of an SI base unit | Base unit | | |
| grave | mass | | | Base unit | |
| gray | absorbed dose | Derived SI unit with a special name | | | |
| hectare | area | Non-SI unit permitted for use with SI units | | Multiple or submultiple of a base unit | |
| henry | inductance | Derived SI unit with a special name | | | |
| hertz | frequency | Derived SI unit with a special name | Derived metric unit with a special name | | |
| hour | time | Non-SI unit permitted for use with SI units | | | No specified relationship |
| joule | energy | Derived SI unit with a special name | | | |
| joule per mole | energy per amount of substance | Derived coherent SI unit without a special name | | | |
| joule-second | angular momentum | Derived coherent SI unit without a special name | | | |
| katal | catalytic activity | Derived SI unit with a special name | | | |
| kelvin | temperature | Base unit | Base unit | | |
| kilogram | mass | Base unit | Multiple or submultiple of a base unit | | |
| kilogram per cubic metre | density | Derived coherent SI unit without a special name | Derived coherent metric unit without a special name | | |
| kilometre per hour | velocity | | | | No specified relationship |
| litre | volume | Non-SI unit permitted for use with SI units | | Multiple or submultiple of a base unit | |
| lumen | luminous flux | Derived SI unit with a special name | | | |
| lumen second | luminous energy | Derived coherent SI unit without a special name | | | |
| lux | illuminance | Derived SI unit with a special name | | | |
| lux second | luminous exposure | Derived coherent SI unit without a special name | | | |
| maxwell | magnetic flux | | Derived metric unit with a special name | | |
| metre per second squared | acceleration | Derived coherent SI unit without a special name | Derived coherent metric unit without a special name | | |
| metre squared per second | angular momentum | Derived coherent SI unit without a special name | Derived coherent metric unit without a special name | | |
| metre | length | Base unit | Multiple or submultiple of a base unit | | |
| microgram | mass | Multiple or submultiple of an SI base unit | Multiple or submultiple of a base unit | | |
| minute | time | Non-SI unit permitted for use with SI units | | | No specified relationship |
| minute of arc | angle | Non-SI unit permitted for use with SI units | | | |
| mole | amount of substance | Base unit | | | |
| neper | level | Non-SI unit permitted for use with SI units | | | |
| newton | force | Derived SI unit with a special name | | | |
| newton-metre | torque | Derived SI metric unit without a special name | | | |
| newton-second | impulse or momentum | Derived coherent SI unit without a special name | | | |
| oersted | magnetic field strength | | Derived SI unit with a special name | | |
| ohm | electric resistance | Derived SI unit with a special name | | | |
| pascal | pressure | Derived SI unit with a special name | | | |
| phot | illuminance | | Derived coherent metric unit without a special name | | |
| poise | dynamic viscosity | | Derived metric unit with a special name | | |
| radian | angle | Derived coherent SI unit with a special name | | | |
| radian per second | angular frequency | Derived coherent SI unit without a special name | | | |
| rayleigh | photon flux | | | No specified relationship | |
| roentgen | kerma of X-rays and gamma rays | | Derived coherent metric unit without a special name | | |
| roentgen equivalent man | radiation dose equivalent | | Derived coherent metric unit without a special name | | |
| second | time | Base unit | Base unit | | |
| siemens | electric conductance | Derived SI unit with a special name | | | |
| sievert | radiation dose equivalent | Derived SI unit with a special name | | | |
| skot | luminance | | | No specified relationship | |
| square kilometre | area | Derived SI unit without a special name | Derived coherent metric unit without a special name | | |
| square metre | area | Derived coherent SI unit without a special name | Derived coherent metric unit without a special name | | |
| statcoulomb | electric charge | | Derived metric unit with a special name | | |
| statvolt | potential difference | | Derived metric unit with a special name | | |
| steradian | solid angle | Derived coherent SI unit with a special name | | | |
| stilb | luminance | | Derived coherent metric unit without a special name | | |
| stokes | kinematic viscosity | | Derived metric unit with a special name | | |
| tesla | magnetic field strength | Derived SI unit with a special name | | | |
| tonne | mass | Non-SI unit permitted for use with SI units | | Multiple or submultiple of a base unit | |
| torr | pressure | | | No specified relationship | |
| volt | potential difference | Derived SI unit with a special name | | | |
| watt | power | Derived SI unit with a special name | | | |
| watt-second | energy | Derived coherent SI unit without a special name | | | |
| weber | magnetic flux | Derived SI unit with a special name | | | |

## History of the metric system

History of the metric system – the metric system developed from a decimal system of measurement adopted by France after the French Revolution.

### Chronological history of the metric system

Principal dates in the development of the metric system include:
* 1792 – Initiation of a decimal system of measurement by the French Revolutionary Government
* 1799 – The Metre des archives and kilogram des archives become the standards for the metric system.
* 1861 – Concept of unit coherence introduced by Maxwell – the base units were the centimetre, gram and second.
* 1875 – Under the Convention of the metre, a new body the General Conference on Weights and Measures (CGPM) was set up and given responsibility for the new international prototype of the kilogram and international prototype of the metre which replaced the old French copies as the definitive prototypes of the metre and the kilogram.
* 1881 – International Electric Congress agrees on standards for electrical units (formalised in 1893)
* 1921 – Convention of the metre extended to cover all physical units of measure
* 1960 – The CGPM published the metric system under the name "International System of Units" (SI) a coherent system of units based on the kilogram, metre, second, ampere and kelvin.

### History of metrication

History of metrication – metrication is the process by which legacy, national-specific systems of measurement were replaced by the metric system.
* Metrication in Australia
* Metrication in Barbados
* Metrication in Canada
* Metrication in Chile
* Metrication in Guatemala
* Metrication in Hong Kong
* Metrication in India
* Metrication in Ireland
* Metrication in Jamaica
* Metrication in New Zealand
* Metrication in Peru
* Metrication in Sweden
* Metrication in the United Kingdom
** British Metrication Board
* Metrication in the United States
** Plan for Establishing Uniformity in the USA, Thomas Jefferson's report (1790) which included a proposal for decimal system based on a "decimal foot"
** United States Metric Board

### Historical metric system variants

Four variants of the metric system that predate the introduction of SI (1960) are described in varying levels of detail:
* MKS system of units formed the basis for SI.
* Centimetre–gram–second system of units was the principal variant of the metric system that evolved in stages until it was superseded by SI.
* Gravitational metric system was a little-used variant of the metric system that normalised the acceleration due to gravity.
* Metre–tonne–second system of units was a variant of the metric system used in French and Russian industry between the First and Second World Wars.
Between 1812 and 1839 France used a quasi-metric system:
* Mesures usuelles

### History of metric units

* History of the metre

## Politics of the metric system

Prior to 1875 the metric system was controlled by the French Government. In that year, seventeen nations signed the Metre Convention and the management and administration of the system passed into international control.
* Metre Convention describes the 1875 treaty and its development to the modern day. Three organisations, the CGPM, CIPM and BIPM were set up under the convention.
* General Conference on Weights and Measures (Conférence générale des poids et mesures or CGPM) – a meeting every four to six years of delegates from all member states.
* The International Committee for Weights and Measures (Comité international des poids et mesures or CIPM) – an advisory body to the CGPM consisting of prominent metrologists.
* The International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM) – an organisation based at Sèvres, France that has custody of the international prototype of the kilogram, provides metrology services for the CGPM and CIPM, houses the secretariat for these organisations and hosts their formal meetings.
Both the European Union and the International Organization for Standardization have issued directives/recommendations to harmonise the use of units of measure. These documents endorse the use of SI for most purposes.
* European units of measurement directives
* ISO/IEC 80000

## Future of the metric system

* 2019 revision of the SI – changes in the definitions of the International System of Units, which are based on defined values of physical constants, in use since May 2019.

## Metrication groups and authorities

* US Metric Association (USA, 1916–current)
* Metric Commission (Canada, 1971–1985)
* Metrication Board (United Kingdom, 1969–1981)

## Metric system publications

* Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States text (Thomas Jefferson 1790)

## Persons influential in the metric system

* Simon Stevin (1548–1620)
* Gabriel Mouton (1618–1694)
* Marquis de Condorcet (1743–1794)
* Pierre Méchain (1744–1804)
* Pierre-Simon Laplace (1749–1827)
* Jean Baptiste Joseph Delambre (1749–1822)
* Adrien-Marie Legendre (1753–1834)
* James Clerk Maxwell (1831–1879)
* William Thomson, 1st Baron Kelvin (1824–1907)
* Giovanni Giorgi (1871–1950)